Making _cat/allocation API use indexLevelStats

[gargharsh3134]

Description

Backporting #15292

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

✅ Gradle check result for 9ecc1da: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.65%. Comparing base (8cf7f92) to head (9ecc1da).
Report is 6 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...ensearch/rest/action/cat/RestAllocationAction.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #16053      +/-   ##
============================================
- Coverage     71.68%   71.65%   -0.04%     
+ Complexity    64650    64639      -11     
============================================
  Files          5277     5277              
  Lines        302465   302466       +1     
  Branches      43998    43998              
============================================
- Hits         216831   216718     -113     
- Misses        67525    67627     +102     
- Partials      18109    18121      +12     

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